This invention relates to a viscosity measurement and more particularly, to an in-situ oil viscosity measurement for an internal combustion engine.
Internal combustion engine designs have been continually improved to reduce weight, increase fuel economy, increase power output, and at the same time meet environmental emission guidelines. Long term, reliable engine operation requires high quality lubricants, for example, an engine oil, transmission fluid, etc., that are able to meet the strenuous demands of the newer, more efficient engines. In general, an engine oil must be thin enough when first starting the engine to allow for sufficient cranking speed, and the oil must then be able to flow immediately to lubricate vital engine components. Most of the engine wear occurs at start-up before the oil can reach all the engine parts. After the engine has reached its desired operating temperature, the oil must not become too thin as to be unable to provide adequate engine lubrication. After the engine is running, the oil is circulated to the engine components and functions to prevent metal-to-metal contact between the various moving parts.
Thus, the engine oil must be sufficiently thin that it can readily flow to all of the areas of the engine requiring lubrication and sufficiently thick that it is able to form a film of oil over the parts being lubricated to prevent metal-to-metal contact. The viscosity of the oil is a measure of its resistance to flow. The viscosity of the oil should be high enough to maintain the desired oil film, however, if the oil viscosity is too high, it can impede the flow of the oil and also, add excess fluid friction.
The viscosity of engine oil remains relatively constant while exposed to relatively constant environmental conditions, however, an internal combustion engine presents a constantly changing environment in which the engine oil must function. First, the engine experiences a wide range of operating temperatures, and the viscosity of engine oil changes with wide fluctuations of temperature. Further, the viscosity of the engine oil changes as it becomes contaminated. Dirt, oxidation and sludge increase the viscosity of the oil while fuel dilution reduces the viscosity. Oil with too low of a viscosity may breakdown and lose strength at higher engine temperatures. Oils with too high a viscosity may not pump through the engine quick enough to lubricate engine components properly at lower engine temperatures.
To ensure high-performance of modern engines, it is important to change the oil of an engine when the quality of the oil has become degraded. Automobile manufacturers generally provide schedules for changing oil based on estimated service conditions and hours or miles of vehicle use. These schedules are generally not very accurate, due to the wide range of service conditions experienced by individual vehicles. At best, the schedules are very conservative and their inaccuracy may result in engine oil being changed either too soon or too late, with regard to the actual quality of the oil. If changed too soon, the discarded oil is unnecessarily wasted. If changed too late, excessive wear or damage to the engine may result.
One way to monitor the quality of an oil is to measure its viscosity, whereby a change in viscosity provides an indication of when the oil should be changed. In this regard, an increase in viscosity indicates a thickening of the oil, such as may be due to the accumulation of contaminants. Likewise, a decrease in viscosity indicates viscosity breakdown. Prior systems which have been used to measure viscosity have required complicated devices that are best suited to use in a laboratory. Accordingly, obtaining a measurement of viscosity required that a sample of the oil be withdrawn and taken to the lab for measurement with the device. Such systems are thus cumbersome and impractical for determination of viscosity on a regular basis.
Other known devices and techniques that are designed for field measurements are either too expensive or lack the ruggedness to be used in-situ with an internal combustion engine.
Thus, there is a need for an economical, real time, in-situ device and process for measuring engine oil viscosity to detect potentially adverse engine operating conditions.
The present invention provides a device that may be used to determine the viscosity of a liquid while immersed directly in the liquid. The device has particular utility for use in determining the viscosity of lubricants in an automobile, such as engine oil and transmission fluid, however the device may be used to determine the viscosity of other liquids in other environments as well. The device operates by measuring or sensing the rate of temperature change of the liquid in response to heat added to the liquid (i.e. the speed at which heat travels through the liquid). Specifically, the device takes advantage of the relationship of the viscosity of a liquid with the convective component of heat. Because heating of a liquid generally produces both convective and conductive heating components, accuracy of the viscosity measurement will be improved if the influence of the conductive components of heat can be minimized.
In accordance with the present invention, a device is provided having a heater for heating a liquid and a temperature sensor for detecting a change in temperature of the liquid in response to heating by the heater. The change in temperature with time is used to determine the viscosity of the liquid. In an exemplary embodiment, the device further includes a housing having a channel section and the heater and temperature sensor are disposed within the channel section and are spaced apart a distance which minimizes conductive flow effects from the heater on the temperature sensor. The channel section is sized to optimize the response time of the temperature sensor with respect to sensing temperature change of the liquid due to heating by the heater. In another aspect of the invention, the channel section is sized to minimize turbulent flow of the liquid near the temperature sensor.
In another exemplary embodiment, the device further includes a controller in communication with the temperature sensor whereby the controller may receive a signal from the temperature sensor corresponding to a measured temperature of the liquid. The controller may be integral with the device or it may be a part of a system with which the device is used, such as an automobile. The controller may be configured to calculate an index value corresponding to the change in temperature with time. The index value may either be compared to a previously stored value for evaluation of the relative change in viscosity, or it may be used to determine an instantaneous value of viscosity for the liquid. Thus, in one aspect, the device may be used to detect a relative change in the viscosity of a liquid, to provide an indication when the quality of a liquid has changed to a point that the liquid should be replaced. In another aspect, the device may be used in conjunction with known viscosity values which have been correlated to the rate of temperature change of the liquid to thereby determine an actual viscosity of the liquid.
In yet another aspect of the invention, a method for determining the viscosity of a liquid includes the steps of heating the liquid, sensing the temperature of the liquid at a first time, sensing the temperature of the liquid at a second time, and integrating the change in sensed temperature with time.